The authors note that the second sentence in the Acknowledgments, “This work was supported by National Institutes of Health (NIH) Grants GM059273 (to A.G.P.) and GM062270 (to L.S.)” should instead appear as “This work was supported by National Institutes of Health (NIH) Grants GM059273 (to A.G.P.) and GM062270 (to L.S.) and by National Science Foundation Grant MCB-0918535 (to B.H.).”

Researchers report trends in emissions of nitrogen oxides in the United States over the past decade. The results suggest challenges to meeting future air quality standards for ozone, according to the authors.